Revert "switch from warning to debug when upstream labels are empty (#724)"

[lucacome]

Proposed changes

This reverts commit d3f9f94.

The commit changed the level for all labels and not just upstream, but in any case, we don't want to suppress the warnings, we want to notify the user of the library that the labels are not configured correctly.

If for some reason the warning wants to be ignored, it's always possible to set the level on the client side.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 3.86%. Comparing base (95324ec) to head (91e9952).

Files	Patch %	Lines
collector/nginx_plus.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #781   +/-   ##
=====================================
  Coverage   3.86%   3.86%           
=====================================
  Files          5       5           
  Lines       1319    1319           
=====================================
  Hits          51      51           
  Misses      1268    1268           

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